Urea is an important raw material for a nitrogen-based fertilizer, one of three major elements essential for the growth of plants, and has supported food production in the world from old times. Even now, increased fertilizer production corresponding to increased food production accompanying population increase in the emerging countries of the world is required, and the construction of a urea synthesis apparatus based on a urea synthesis method having excellent reliability and productivity is an important problem.
Here, the reliability mainly refers to the property of protecting the metal surface inside this apparatus from ammonium carbamate produced as a reaction intermediate when urea is synthesized from ammonia and carbon dioxide, having strong corrosiveness to metal. On the other hand, the productivity refers to not only increasing the yield of the reaction of ammonia and carbon dioxide but a reduction in total urea production cost including a reduction in apparatus construction cost and a reduction in running cost with improved operating conditions.
Technical development for increasing such reliability and productivity has been mainly addressed from three viewpoints. The first is a chemical viewpoint that a passivation film is formed on the metal surface of the apparatus to increase corrosion resistance. The second is a material viewpoint that corrosion resistance to ammonium carbamate is enhanced by an improvement in the apparatus material itself. The third is a process viewpoint that improvements in corrosion resistance and reaction yield by improvements in the production process and the apparatus are promoted. But, these are in a conflicting relationship with each other, and a balanced urea synthesis method is required even now.
First, various techniques for forming a passivation film on a metal surface have been studied such as methods using oxidants such as oxygen, nitric acid, hydrogen fluoride, hydrogen peroxide, ammonium nitride, and ozone as disclosed in U.S. Pat. Nos. 2,680,766, 2,727,069, 3,137,724, British Patent No. 1,153,107, European Patent No. 0096151, and the like, all of which are hereby incorporated by reference. As a result, a method using oxygen as an oxidant is adopted, that is, a method of mixing oxygen with carbon dioxide and ammonia, starting raw materials, to form about 1 to 3 nm of a passivation film comprising hydroxides of Fe and Cr and an oxide of Cr at high temperature and high pressure.
But, when the amount of oxygen fed as an oxidant is too large, inert gases increase, and the H/C (H2O/CO2) ratio described later increases, and the reaction yield decreases. On the other hand, an energy-consuming scrubber for finally separating and removing oxygen from unconverted raw materials and other volatile components, and a hydrogen combustion removal apparatus for preventing a risk that a trace amount of hydrogen contained in the urea product and oxygen are mixed are also required. In this manner, a problem of oxygen is that with an increase in feed, the corrosion resistance is increased, and on the other hand, the running cost is adversely affected.
Therefore, formerly, as noted in austenitic stainless steel S31603 series described in U.S. Pat. No. 2,680,766, development has been advanced in the direction of increasing the corrosion resistance of stainless steel itself. With this, improvements in workability required for apparatus production have also been made. As disclosed in British Patent No. 1,192,044, European Patent No. 0096151, International Publication No. WO 95/00674, British Patent No. 775,933,International Publication No. WO 03/018861, Japanese Patent Laid-Open No. 2003-301241, and the like, all of which are hereby incorporated by reference, improvements are made paying attention to the effects of Cr, Ni, Mo, and N. Particularly, International Publication No. Wo 03/018861 proposes austenite-ferrite duplex stainless steel having a Cr content of 28 to 35 wt %, a Ni content of 3 to 10 wt %, a Mo content of 1.0 to 4.0 wt %, and a N content of 2.0 to 0.6 wt %, suggesting that the construction of a urea synthesis plant that hardly requires corrosion prevention oxygen is possible.
But, a problem is that the material price of austenite-ferrite duplex stainless steel having excellent corrosion resistance and high Cr content in this manner is high and the construction cost of a urea plant increases, and the construction of a urea synthesis apparatus having both reliability and productivity is difficult.
Therefore, as disclosed in British Patent No. 1,341,497, British Patent No. 1,287,710, Japanese Patent Laid-Open No. 53-14993, Japanese Patent Laid-Open No. 56-131558, Japanese Patent Laid-Open No. 60-209555, Japanese Patent Laid-Open No. 10-182587, Japanese Patent Laid-Open No. 11-180942, and the like, all of which are hereby incorporated by reference, improvements in the production process and the apparatus have also been performed. Particularly, effectively recycling unreacted products by the introduction of a stripper, and providing to the condenser a urea synthesis function similar to that of the reactor increase the reaction yield, can make the production equipment small, and can contribute to the enhancement of productivity.
When the conventional art as described above is comprehensively considered, it is summarized into two types of urea synthesis methods. One uses expensive stainless steel having high Cr content and excellent corrosion resistance, and therefore, the apparatus can be operated at extremely low oxygen concentration, but a large burden is placed on the construction cost. For example, when austenite-ferrite duplex stainless steel having a Cr content of 28 to 35 wt %, a Ni content of 3 to 10 wt %, a Mo content of 1.0 to 4.0 wt %, and a N content of 0.2 to 0.6 wt % as disclosed in the above-described International Publication No. WO 03/018861 is used, the apparatus, including the stripper in which corrosion proceeds most easily, can be operated in a state in which oxygen is hardly required. But, the stainless steel is expensive, and a large burden is placed on the construction cost. The other uses general-purpose stainless steel such as S31603, and therefore, the burden of construction cost decreases, but the reaction yield is low, accessory equipment such as a scrubber for separating oxygen and a hydrogen combustion removal apparatus for removing hydrogen is required, and the construction cost and the running cost increase. For example, U.S. Pat. No. 2,727,069 discloses that in a urea synthesis plant using S31603 series general-purpose stainless steel, an oxygen concentration as much as about 1,000 to 30,000 ppm with respect to carbon dioxide is required. In addition, British Patent No. 1,341,497also discloses that in a similar urea synthesis plant using S31603 series stainless steel, an oxygen concentration of 1,000 to 25,000 ppm with respect to carbon dioxide is required in the stripper, and an oxygen concentration of 300 to 10,000 ppm with respect to carbon dioxide is also required in the condenser.
In U.S. Pat. No. 3,137,724, it is described that a urea synthesis plant using S31603 series stainless steel can be operated at an oxygen concentration of 100 to 500 ppm with respect to carbon dioxide, and in British Patent No. 1,192,044, it is described that urea synthesis plant using S31260 series stainless steel can be operated at an oxygen concentration of 100 to 500 ppm with respect to carbon dioxide. But, the oxygen concentration is oxygen concentration in the urea synthesis tower in which the progress of corrosion is slower than in the stripper, and there is no description regarding a stripper.
In addition, European Patent No. 0096151 also proposes that S31260 series stainless steel is used, and the apparatus can be operated with feed at an oxygen concentration of 200 to 2,000ppm with respect to carbon dioxide from the lower portion of the stripper. But, hydrogen peroxide is mixed into the upper portion of the reactor R in the synthesis system, oxygen is mixed into the lower portion, and hydrogen peroxide is mixed into the synthesis gas from the reactor R in the stripper S1. Oxygen is introduced into the bottom of the stripper S2, but it is necessary to introduce hydrogen peroxide from the top in order to compensate for an increase in the amount of gas due to stripping and a relative decrease in oxygen partial pressure at the top. The gas exiting the stripper S2 is condensed in the condenser C, and only the liquid phase portion is fed to the reactor, and the gas phase portion is vented from the upper portion of the condenser C. The oxygen introduced from the bottom of the stripper S2 and the oxygen in the hydrogen peroxide introduced from the upper portion transition to the gas phase portion to be vented, produced in the condenser C. Therefore, the oxygen introduced in a stripper S2 is hardly introduced into the reactor R, and therefore, it is necessary to separately introduce oxygen for passivation the metal surface of the reactor. Therefore, in the process disclosed in European Patent No. 0096151, oxygen must be introduced in a plurality of parts.
As described above, in the conventional art, a urea synthesis method satisfying reliability and productivity having a balance of the oxygen concentration required for corrosion resistance, the material of stainless steel, and an efficient production process is not found.